There have been many cases at a construction or building site where the earth must be deeply excavated to form a hole having a depth which is too long relative to its diameter. For example, there have been cases for excavating the earth to form a hole in which an anchor supporting a steel tower is embedded, a hole in which a water purifier tank is embedded, a hole for ground making and a hole for well sinking. In such cases, the hole should generally have a depth which is too long, e.g. ranging from 15 m to 20 m, relative to its diameter, e.g. about 5 m.
In deep excavating work, there is conventionally employed a deep excavator having a telescopic mechanism comprising a stretchable arm arrangement wherein a clamshell bucket (hereinafter referred to as a bucket) is coupled with the tip end of a top arm of the stretchable arm arrangement. In the conventional deep excavator, the stretchable arm arrangement is typically fixed to the tip end of the boom and has at least two stages of arms in which the bucket suspended from the top arm is hung to reach the bottom of the hole.
In the conventional stretchable mechanism for extending or contracting each arm, a wire or chain is entrained around or extended between each arm whereby each arm is extended and contracted synchronously with one another by such wire or chain. In such a mechanism, it is possible to smoothly extend or contract each arm of the stretchable arm arrangement but the wire or chain must be entrained around or extended to each arm, which makes the arrangement of the wire or chain complex. Furthermore, since the wire or chain for contraction of each arm as well as extension of each arm must be entrained around or extend to each arm, at least two wires or chains are required for one arm, which leads to a complex arrangement of the wires or chains. In such an arrangement, the wires or chains are liable to be exposed outside the stretchable arm arrangement which is not preferable in view of external appearance. There is a likelihood that earth or sand will become stuck to the wires or the chains, which causes abrasion or is troublesome to the mechanism.
Accordingly, there is proposed a mechanism for extending or contracting a stretchable arm arrangement using hydraulic power generated by a single hydraulic cylinder which is incorporated into the stretchable arm arrangement comprising a plurality of telescopic arms. However, in this mechanism, the amount of extension of the stretchable arm arrangement is limited and the speed of extension is not increased. To solve these problem, there is further proposed a mechanism having two stretchable arms each having a hydraulic cylinder wherein the hydraulic cylinders are simultaneously operated to thereby extend and contract the entire stretchable arm arrangement. However, if a plurality of hydraulic cylinders are accommodated in the stretchable arm, it is necessary to provide high pressure application hoses on each hydraulic cylinder coupled with each arm, which makes the mechanism complex. Even if a plurality of hydraulic cylinders are used, the stretchable arm arrangement cannot be extended and contracted at high speed.
To solve the problem, there has been proposed a mechanism, for example, as disclosed in Japanese Patent Application Nos. 4-130104 and 4-157331 (and corresponding U.S. Pat. No. 5,375,348) for simultaneously operating a plurality of stretchable arms using a working unit comprising two sets of hydraulic cylinders (two sets of hydraulic cylinder units) which are alternately assembled and arranged in parallel with each other and structured so that cylinder rods thereof are disposed to operate in the opposite direction.
In the working unit, the top and base arms can be extended or contacted by a pair of hydraulic cylinder units in the working unit, and the extending or contracting speeds can be faster. A hydraulic system, i.e. piping system for supplying oil under pressure to both hydraulic cylinder units are supplied from the upper end of the working unit, and oil is collected by a pipe connected to this cylinder rod. Accordingly, the hydraulic conduit connected to the hydraulic cylinder unit is not necessary to be loose inside the telescopic stretchable arm arrangement, which makes the mechanism very simple.
In this mechanism, a long hydraulic conduit is not necessary to be disposed for extending or contracting the stretchable arm arrangement, but an additional hydraulic conduit must be disposed for supplying the oil under pressure to various hydraulic apparatus and instruments such as a clam-shell bucket, a shedding machine and a cutting mechanism. If such hydraulic machines and implements are not operated, a holding operation of the earth or sand and shedding operation cannot be performed even if the stretchable arm arrangement can be extended or contracted, hence the intended working cannot be performed. To loosen the hydraulic conduit, the length of the hydraulic conduit is set to such a length in that the stretchable arm arrangement is extended to the maximum, and the hydraulic conduit must be bent between each arm in the stretchable arm arrangement. The hydraulic conduit (i.e. hose) is formed of flexible synthetic rubber and is arranged between each arm with looseness, whereby the hydraulic conduit can be driven following the extending or contracting operations of the stretchable arm arrangement.
If the long hydraulic conduit is arranged in the stretchable arm arrangement so as to operate the stretchable arm arrangement, the hydraulic conduit per se is bent and deteriorated when used for a long period of time, which causes leakage of the oil under pressure from the hydraulic conduit. Accordingly, it was necessary to inspect the hydraulic conduit periodically so as to maintain or prevent the damage of the hydraulic conduit per se. Since the hydraulic conduit is arranged in the stretchable arm arrangement with looseness, this causes an increase of the weight of the hydraulic conduit and also causes the increase of the overall weight of the stretchable arm arrangement.
As mentioned above, the aforementioned conventional deep excavator has such a drawback in that the hydraulic conduit was needed since the hydraulic machine and instrument such as the clam-shell bucket or the shedding machine are coupled with the top arm of the stretchable arm arrangement comprising a plurality of arms, and oil under pressure must be supplied from the chassis to the hydraulic machine and instrument, which makes the hydraulic conduit difficult to handle. Accordingly, it is desired to develop a deep excavator capable of supplying oil under pressure from the base arm to the top arm of the stretchable arm arrangement without using the hydraulic conduit, and also capable of preventing leakage of the oil under pressure from the stretchable arm arrangement even if the stretchable arm arrangement is extended or contracted.